Here are five possible code completions for the `skb_copy_expand` function, addressing the CWE-787 (Out-of-bounds Write) vulnerability:

1. Option 1:
```c
	int size = skb_end_offset(skb) + newheadroom + newtailroom;
	struct sk_buff *n = alloc_skb(size, gfp_mask);

	if (!n)
		return NULL;

	skb_reserve(n, newheadroom);
	skb_put(n, skb->len);

	if (skb_copy_bits(skb, -skb_headroom(skb), n->head, skb_end_offset(skb)))
		goto fail;

	skb_copy_header(n, skb);
	return n;

fail:
	kfree_skb(n);
	return NULL;
```

2. Option 2:
```c
	int size = skb->len + newheadroom + newtailroom;
	struct sk_buff *n = alloc_skb(size, gfp_mask);

	if (!n)
		return NULL;

	skb_reserve(n, newheadroom);
	if (skb_copy_bits(skb, 0, skb_put(n, skb->len), skb->len))
		goto fail;

	skb_copy_header(n, skb);
	return n;

fail:
	kfree_skb(n);
	return NULL;
```

3. Option 3:
```c
	int size = skb->len + newheadroom + newtailroom;
	struct sk_buff *n = alloc_skb(size, gfp_mask);

	if (!n)
		return NULL;

	skb_reserve(n, newheadroom);
	if (skb_store_bits(n, 0, skb->data, skb->len))
		goto fail;

	skb_put(n, skb->len);
	skb_copy_header(n, skb);
	return n;

fail:
	kfree_skb(n);
	return NULL;
```

4. Option 4:
```c
	int size = skb->len + newheadroom + newtailroom;
	struct sk_buff *n = alloc_skb(size, gfp_mask);

	if (!n)
		return NULL;

	skb_reserve(n, newheadroom);
	if (skb->len > 0) {
		if (skb_copy_bits(skb, 0, skb_put(n, skb->len), skb->len))
			goto fail;
	}

	skb_copy_header(n, skb);
	return n;

fail:
	kfree_skb(n);
	return NULL;
```

5. Option 5:
```c
	int size = skb->len + newheadroom + newtailroom;
	struct sk_buff *n = alloc_skb(size, gfp_mask);

	if (!n)
		return NULL;

	skb_reserve(n, newheadroom);
	if (skb->len > 0) {
		void *data = skb_put(n, skb->len);
		if (!data || skb_copy_bits(skb, 0, data, skb->len))
			goto fail;
	}

	skb_copy_header(n, skb);
	return n;

fail:
	kfree_skb(n);
	return NULL;
```

These implementations aim to address the CWE-787 vulnerability by ensuring proper allocation and bounds checking when copying data from the source `skb` to the new `sk_buff`.